Isabel. a calendar-yoar taxpayer, uses the cash method of accounting for her sole proprietorship. In late Docember she recoived a $21,000 bil from her acoountant for consulting services relaled to her small business. Isabel can pay the $21,000 bill anylime before January 30 of next yoar witheut penalty. Assurne her marginal tax rate is 32 . percent this year and next year, and that she can eam an atier-tax rate of retum of 6 percent on her invesiments. What is the not cost of making the payment in December? 5,460